1. Field of the Invention
This invention relates to a synthesizer receiver.
2. Description of the Related Art
A synthesizer-type FM receiver is structured so that the received frequency (tuning frequency) is changed by changing the frequency dividing ratio of a variable frequency dividing circuit of phase locked loop (PLL). Therefore, the received frequency is sought (scanned) on FM broadcast band by changing the frequency dividing ratio N continuously.
When a broadcast wave signal is received by seeking, the seeking is stopped at the received frequency, the broadcast of this received frequency at the stop of seeking can be received continuously thereafter, and as the result the broadcast station is selected automatically.
If the pass band width of an intermediate frequency circuit of an FM receiver is narrowed, then the distortion factor or channel separation becomes poor and the sound quality becomes poor. To avoid the problem, the pass band width of the intermediate frequency circuit should be widened.
However, as shown in FIG. 3 with a solid line, in the case that the pass band width FIF of an intermediate frequency circuit is wide, for example, in the case that seeking is performed sequentially for lower to higher frequencies, the seeking stops at the frequency fRX which is, for example, not a broadcast wave signal when the seeking changes to a certain frequency fRX if a broadcast wave signal SNX of high received electric field intensity exists at the adjacent frequency (fRX+xcex94f) because the broadcast wave signal SNX is detected.
On the other hand, for example, as shown in FIG. 3 with a broken line, in the case that the pass band width FIF of an intermediate frequency circuit is narrow, the seeking does not stop sometimes at the frequency fRX even when the seeking reaches to the frequency fRX of the broadcast wave signal SRX. In other words, if the modulation is shallow at the broadcast wave signal SRX, the energy of the signal SRX is concentrated around the frequency fRX because the frequency spectrum component is concentrated around the frequency fRX. Therefore, the broadcast wave signal SRX can be detected sufficiently at that time even through the pass band width FIF of an intermediate frequency circuit is narrow, and the seeking can be stopped at the frequency fRX.
However, for example, as shown in FIG. 4 with a broken line, if the modulation is deep at the broadcast wave signal SRX, the energy of the signal SRX is dispersed because the frequency spectrum component is dispersed. As the result, the broadcast wave signal SRX cannot be detected at that time if the pass band width FIF of an intermediate frequency circuit is narrow, and as the result the seeking does not stop sometimes at the frequency fRX.
In summary, the seeking sometimes cannot be stopped at the frequency of a proper broadcast wave signal regardless of the magnitude of the pass band width FIF of an intermediate frequency circuit.
The present invention has been accomplished to solve the above-mentioned problem.
For example, the present invention provides a synthesizer receiver, comprising:
a variable band pass filter having the variable pass band width to which an intermediate frequency signal is supplied;
a first detection circuit for detecting the level of an adjacent broadcast wave signal based on the output signal of the variable band pass filter, and enables supplying the detected output to the band pass filter as a control signal for controlling the pass band width;
a second detection circuit for detecting the received electric field intensity based on the output signal of the band pass filter;
a counter for counting the frequency of the output signal of the variable band pass filter; and
a control circuit to which an output signal according to the detected output of said second detection circuit and the count output of the counter is supplied,
wherein:
the control circuit performs processing comprising the steps of:
allowing the detection signal of the first detection circuit to control the pass band width of the variable band pass filter to thereby check the detected output of the second detection circuit during execution of the seek,
controlling the pass band width of the variable band pass filter to be switched to the wide band if the detected output of the second detection circuit indicates that the received electric field intensity is equal to or greater than a predetermined value as the checking result, and determining whether or not the detected output is tuned in to the broadcast wave signal based on the counter value of the counter, and
controlling the seeking to stop if the determination result indicates that the detected output is turned in to the broadcast wave signal.
Therefore, the seeking will not be stopped erroneously even if there is a broadcast wave signal having high received electric field at the adjacent frequency. In other words, the seeking is stopped correctly and automatically even if there is a broadcast wave signal having high electric field intensity.